This invention relates to electronic devices for receiving a plurality of radiated electromagnetic signals, filtering a selectable channel of frequencies from the received signals, and demodulating the signals of the selected filtered channel. More particularly, the invention relates to television receivers.
Television receivers of the prior art include a radio frequency (RF) section and an intermediate frequency (IF) section. The RF section includes RF filters which are tuned to coarsely filter a band of channels centered about a manually selected channel. The output of the RF filter couples to the input of an RF amplifier. Typically, total gain through the RF section is at least 20 dB to 30 dB. This gain increases the amplitude of signals within the selected channel and additionally makes the noise figure of the system essentially independent of subsequent elements in the receiver. The output of the RF amplifier couples to one input of a mixer, while a second input of the mixer receives mixing signals of a selectable frequency. The selectable frequency is generated such that the selected channel is frequency shifted to approximately 45 MHZ. The output of the mixer couples to a channel selection filter which provides a relatively high impedance path for frequencies outside of the selected channel, and a relatively low impedance path for signals inside the selected channel. Signals at the output of the channel selection filter are therefore primarily comprised of frequencies within the selected channel.
Each television channel contains audio information, video information, and frame synchronizing information. The output of the channel selection filter couples to an audio demodulator which separates the audio information from the selected channel; and the output of the audio demodulator couples to a speaker which generates audible sounds. Similarly, the output of the channel selection filter couples to a video processing unit which separates the video and frame synchronizing information from the selected channel; and the output of the video processor couples to a picture tube which converts the video and frame synchronizing information to pictures.
As described above, a basic function which all television receivers perform is to frequency shift the selected channel from an RF frequency to a predetermined IF frequency by a mixing device. This mixing operation has been performed in the past by a variety of nonlinear devices. These devices include vacuum tubes, diodes, MOSFET transistors, and bipolar transistors. However, the prior art mixers also generate output terms which are proportional to their inputs cubed or raised to higher order odd powers. In a television receiver, these odd power terms may generate interfering signals in the desired channel. For example, such interfering signals are generated when signals are present in channels on one side of the selected channel which are one and two channels removed from the selected channel. This phenomena is known as intermodulation distortion. Similarly, the cubic terms and higher order odd power terms generate interfering signals in the desired channel when a carrier with amplitude modulation is present in anyone of the undesired channels. This phenomena is known as cross modulation distortion. The frequencies which are generated in a desired channel as a result of intermodulation distortion or cross modulation distortion cannot be separated from the information signals lying therein. Thus, as the magnitude of the interfering frequencies increases, perceptible picture distortion or sound distortion occurs.
A principal advantage of the television receiver herein disclosed is that it has greatly reduced cross modulation and intermodulation distortion. This is accomplished to a large extent by means of a unique RF mixer. The mixer utilizes a MESFET transistor which has almost perfect square law current-voltage characteristics. Since the MESFET mixer has almost perfect square law characteristics, the mixer introduces extremely small distortion into the system. In particular, the MESFET mixer handles an interfering signal level of greater than +6 dBm on its output with less than 1% cross modulation distortion and -40 dB intermodulation distortion products.
The square law current-voltage characteristics of a MESFET transistor and their application to reduce intermodulation and cross modulation distortion in a television receiver are not taught by the prior art. In the past, MESFET devices were used primarily to achieve high speed operation. For example, they were utilized in pico-second digital switching circuits. See, for example, a paper by Cahen et al entitled "A Subnanosecond Switching Circuit" which was presented at the IEEE International Solid State Circuits Conference on Feb. 14, 1974 at Orsay, France. See also, for example, a paper entitled "X and KU Band Amplifiers with GaAs Schottley Barrier Field Effect Transistors" by Weiner Baechtold in the IEEE Journal of Solid State Circuits Volume, SC-8, No. 1, February 1973. High speed digital switching is combined with high speed linear amplification in a high speed pulse amplitude modulation device that is described in the paper entitled "Performance of Dual Gate GaAs MESFETS as Gain Controlled Low Noise Amplifiers and High Speed Modulators" by Liechti in the IEEE Transactions on Microwave Theory and Techniques Vol. MIT-23, No. 6, June 1975. However, all of the devices in the above cited references only utilize MESFETS for high speed.
The television receiver herein disclosed is also novel in that it includes a unique combination of surface wave device technology with MESFET device technology in the RF-IF section. A MESFET mixer provides frequency shifting while channel selectivity is provided by a single low loss surface acoustic wave device (i.e. SWD) bandpass filter. This SWD filter has a sharp passband--stop band transition. For example, signals 1.5 MHZ above the sound carrier in the passband are attenuated by greater than 65 dB. Conversely, the filter has a low insertion loss for signals in the passband. The preferred embodiment is a unidirectional SWD filter which has an inband insertion loss of less than 3.5 dB.
The SWD filter also has a high center frequency and a passband which is a small percentage of the center frequency. Thus, the SWD filter is readily implemented with piezoelectric material having a relatively low coefficient of coupling. Quartz has such a characteristic, and it is a preferred substrate material. By comparison, prior art television receivers have employed SWD filters but only at a much lower IF frequency of 45 MHZ. Accordingly, their passband was a relatively large percentage of their center frequency and thus the devices needed piezoelectric material having a relatively large coefficient of coupling-such as lithium niobate. Lithium niobate, however, has piezolelectric characteristics which are highly sensitive to temperature change in the 0.degree.-70.degree. C. temperature range. As a result, they require temperature compensation circuitry. By comparison, the piezoelectric characteristics of quartz are relatively insensitive to temperature change in the 0.degree.-70.degree. temperature range.
Accordingly, it is one object of the invention to provide an improved device for receiving signals in selected channels from a plurality of non-overlapping channels.
Another object of the invention is to provide a television receiver having improved reception.
Another object of the invention is to provide a television receiver simultaneously having significantly reduced intermodulation distortion, significantly reduced cross modulation distortion, and improved out-of-band signal rejection.
Another object of the invention is to provide a television receiver having a MESFET mixer for frequency shifting a selected channel to a predetermined high IF frequency.
Another object of the invention is to provide a channel selector for a television receiver which includes a MESFET mixer in combination with an acoustic surface wave device filter for channel selection.
Another object of the invention is to provide a channel selector for a television receiver having a MESFET mixer in combination with a low loss unidirectional surface wave device filter.
Another object of the invention is to provide a channel selector for a television receiver having a MESFET mixer in combination with a surface wave device filter which requires no temperature compensation.